Expression of heterologous DNA sequences in a plant host is dependent upon the presence of an operably linked regulatory element that is functional within the plant host. Choice of the regulatory element sequence will determine when and where within the organism the heterologous DNA sequence is expressed. Where expression in specific tissues or organs is desired, tissue-preferred regulatory elements may be used. Where gene expression in response to a stimulus is desired, inducible regulatory elements are the regulatory element of choice. In contrast, where continuous expression is desired throughout the cells of a plant, constitutive promoters are utilized. Additional regulatory sequences upstream and/or downstream from the core regulatory element sequence may be included in the expression constructs of transformation vectors to bring about varying levels of expression of heterologous nucleotide sequences in a transgenic plant.
Frequently it is desirable to express a DNA sequence constitutively in a plant. For example, increased resistance of a plant to infection by soil- and air-borne pathogens might be accomplished by genetic manipulation of the plant's genome to comprise a constitutive regulatory element operably linked to a heterologous pathogen-resistance gene such that pathogen-resistance proteins are produced in the desired plant tissue.
Alternatively, it might be desirable to inhibit expression of a native DNA sequence within a plant's tissues to achieve a desired phenotype. In this case, such inhibition might be accomplished with transformation of the plant to comprise a constitutive promoter operably linked to an antisense nucleotide sequence, such that expression of the antisense sequence produces an RNA transcript that interferes with translation of the mRNA of the native DNA sequence.
Genetically altering plants through the use of genetic engineering techniques and thus producing a plant with useful traits requires the availability of a variety of promoters. An accumulation of promoters would enable the investigator to design recombinant DNA molecules that are capable of being expressed at desired levels and cellular locales. Therefore, a collection of constitutive promoters would allow for a new trait to be expressed at the desired level in the desired tissue. Thus, isolation and characterization of constitutive regulatory elements that may serve as regulatory regions for expression of heterologous nucleotide sequences of interest in a measured constitutive manner are needed for genetic manipulation of plants.